U.S. patent number 8,187,136 [Application Number 12/237,959] was granted by the patent office on 2012-05-29 for differential gear.
This patent grant is currently assigned to Musashi Seimitsu Industry Co., Ltd.. Invention is credited to Tomoyuki Fujita, Akira Mataga.
United States Patent |
8,187,136 |
Fujita , et al. |
May 29, 2012 |
Differential gear
Abstract
In a differential gear, a differential case includes: a first
differential case on a side ranging from a pinion-shaft
installation part on which a pinion shaft is installed, to a ring
gear; and a second differential case on a side opposite to the side
ranging from a pinion-shaft installation part on which the pinion
shaft is installed, to the ring gear. The first differential case
is integrally molded from only a low-carbon steel containing less
than 0.45% of C, by forging or by forging and cutting. The second
differential case is integrally molded from only a low-carbon steel
containing less than 0.45% of C, by forging or by forging and
cutting. The first differential case and the second differential
case are bonded to each other by welding.
Inventors: |
Fujita; Tomoyuki (Toyohashi,
JP), Mataga; Akira (Toyohashi, JP) |
Assignee: |
Musashi Seimitsu Industry Co.,
Ltd. (Toyohashi-Shi, JP)
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Family
ID: |
40509044 |
Appl.
No.: |
12/237,959 |
Filed: |
September 25, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090088286 A1 |
Apr 2, 2009 |
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Foreign Application Priority Data
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Sep 28, 2007 [JP] |
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2007-254519 |
Sep 22, 2008 [JP] |
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2008-242389 |
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Current U.S.
Class: |
475/230;
74/606R |
Current CPC
Class: |
F16H
48/08 (20130101); Y10T 74/2186 (20150115); F16H
2048/385 (20130101); F16H 2048/085 (20130101); F16H
2048/382 (20130101); F16H 48/40 (20130101) |
Current International
Class: |
F16H
48/06 (20060101) |
Field of
Search: |
;74/606R,607
;475/230,231 ;29/463,525.14 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2000-266162 |
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Sep 2000 |
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JP |
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2006-509172 |
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Mar 2006 |
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JP |
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2004/053357 |
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Jun 2004 |
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WO |
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Other References
Patent Abstracts of Japan 2000-266162 dated Sep. 26, 2000. cited by
other.
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Primary Examiner: Holmes; Justin
Assistant Examiner: Le; Huan
Attorney, Agent or Firm: Ladas & Parry LLP
Claims
The invention claimed is:
1. A differential gear assembly comprising: ring gear (9) to which
a driving force is inputted from a driving source so as to be
rotated; a differential case (2) configured to be rotated together
with the ring gear (9); a pinion shaft (3) installed on the
differential case (2), the pinion shaft (3) being configured to be
rotated together with the differential case (2); pinion gears (21,
22) pivotally mounted on the pinion shaft (3); and side gears (23,
24) configured to be meshed with the pinion gears (21, 22);
wherein: the differential case (2) includes: (i) a first
differential case (2a) on a first side of the differential case and
comprising a pinion shaft installation part (10) on which the
pinion shaft (3) is installed, the ring gear (9) and an area
ranging from the pinion-shaft installation part (10) on which the
pinion shaft (3) is installed to the ring gear (9); and (ii) a
second differential case (2b) on a second side of the differential
case opposite to the first side and not comprising the area ranging
from the pinion-shaft installation part (10) on which the pinion
shaft (3) is installed to the ring gear (9); wherein the first
differential case (2a) is integrally molded from only a low-carbon
steel containing less than 0.45% of carbon, by forging or by
forging and cutting; wherein the second differential case (2b) is
integrally molded from only a low-carbon steel containing less than
0.45% of carbon, by forging or by forging and cutting; and wherein
the first differential case (2a) and the second differential case
(2b) are bonded to each other by welding to form a weld outside of
the area ranging from the pinion-shaft installation part and the
ring gear, said area being seamless.
2. The differential gear assembly according to claim 1, wherein the
first differential case (2a) is integrally molded from only a
low-carbon steel whose content of carbon is between 0.10% and
0.40%, by forging or by forging and cutting.
3. The differential gear assembly according to claim 1, wherein the
second differential case (2b) is integrally molded from only a
low-carbon steel whose content of carbon is between 0.10% and
0.40%, by forging or by forging and cutting.
4. The differential gear assembly according to claim 1, wherein the
first differential case (2a) and the pinion shaft (3) are secured
to each other by caulking.
5. The differential gear assembly according to claim 4, wherein the
pinion shaft (3) is molded from only a low-carbon steel containing
less than 0.45% of carbon.
6. A differential gear assembly comprising: a ring gear (9) to
which a driving force is inputted from a driving source so as to be
rotated; a differential case (2) configured to be rotated together
with the ring gear (9); a pinion shaft (3) installed on the
differential case (2), the pinion shaft (3) being configured to be
rotated together with the differential case (2); pinion gears (21,
22) pivotally mounted on the pinion shaft (3); and side gears (23,
24) configured to be meshed with the pinion gears (21, 22);
wherein: the differential case (2) includes: a first differential
case (2a) on a first side, the first differential case comprising a
pinion shaft installation part (10) on which the pinion shaft (3)
is installed, the ring gear (9), and an area ranging from the
pinion-shaft installation part (10) on which the pinion shaft (3)
is installed, to the ring gear (9); and a second differential case
(2b) on a side opposite to the first side and not comprising the
area ranging from the pinion-shaft installation part (10) on which
the pinion shaft (3) is installed to the ring gear (9); wherein the
first differential case (2a) is integrally molded from only a first
structured steel containing less than 0.60% of Carbon Equivalent,
by forging or by forging and cutting; wherein the second
differential case (2b) is integrally molded from only a second
structured steel containing less than 0.60% of Carbon Equivalent,
by forging or by forging and cutting; and wherein the first
differential case (2a) and the second differential case (2b) are
bonded to each other by welding to form a weld outside of the area
ranging from the pinion-shaft installation part and the ring gear,
said area being seamless.
7. The differential gear assembly according to claim 6, wherein the
first structured steel and the second structured steel are selected
such that a sum of a hot crack sensitivity of the first structured
steel and a hot crack sensitivity of the second structured steel is
not more than 7.
8. The differential gear assembly according to claim 6, wherein the
first differential case (2a) and the pinion shaft (3) are secured
to each other by caulking.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from the prior Japanese Patent Application Nos. 2007-254519 filed
on Sep. 28, 2007, and 2008-242389 filed on Sep. 22, 2008, the
entire contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to a differential gear including: a
ring gear to which a driving force is inputted from a driving
source so as to be rotated; a differential case configured to be
rotated together with the ring gear; a pinion shaft installed on
the differential case, the pinion shaft being configured to be
rotated together with the differential case; pinion gears pivotally
mounted on the pinion shaft; and side gears configured to be meshed
with the pinion gears.
BACKGROUND ART
There has been conventionally known a differential gear used in a
vehicle such as an automobile, which transmits a driving force
(torque) from an engine, through a ring gear, a differential case
installed on the ring gear, a pinion shaft installed on the
differential case, pinion gears installed on the pinion shaft, and
side gears configured to be meshed with the pinion gears. The
pinion shaft, the pinion gears, and the side gears, are arranged
inside the differential case.
As a method of forming such a differential case of the differential
gear, there is known a technique including the steps of: preparing
a first differential case and a second differential case;
integrally molding the first differential case with a ring gear by
forging; integrally molding the second differential case with a
pinion-shaft installation part on which a pinion shaft is
installed, by forging; and assembling the first differential case
and the second differential case by a plurality of bolts (see,
JP2000-266162-A).
In another method of forming such a differential case of the
differential gear, a first differential case and a second
differential case are firstly prepared. The first differential case
is molded by pressing, and the second differential case having a
pinion-shaft installation part is molded by cold rolling. Then, a
ring gear is separately molded. Thereafter, the first differential
case and the second differential case are bonded by welding, and
the second differential case and the ring gear are bonded by
welding (see, JP2006-509172-A).
DISCLOSURE OF THE INVENTION
In the technique of Patent Document 1 in which the first
differential case and the second differential case are assembled by
a plurality of bolts, a torque from the ring gear is transmitted to
the second differential case via a plurality of bolt installation
parts. Thus, it is necessary to ensure durability of the plurality
of bolt installation parts of the first differential case and the
second differential case, and thus it is necessary to increase a
thickness of a portion around each bolt installation part. This
disadvantageously invites an increase in weight of the differential
case. In addition, since a plurality of bolts are required, there
is a problem in that the number of components is increased, which
results in cost increase.
On the other hand, in the molding method of Patent Document 2, no
bolt is used. Thus, the aforementioned problems do not arise.
However, since a welding part between the first differential case
and the second differential case, and a welding part between the
second differential case and the ring gear, are located in an area
through which a driving force from an engine is transmitted (an
area between the pinion-shaft installation part and the ring gear),
there is a possibility that the welding parts, which are relatively
poor in strength, are cracked when a torque is transmitted. Namely,
the differential case has a durability problem.
The present invention has been made to solve the above problems. It
is an object of the present invention to provide a differential
gear having a desired durability, in which a first differential
case and a second differential case are secured to each other not
by means of bolts so as to reduce a weight, and the number of
components are reduced so as to reduce a cost.
The present invention is a differential gear comprising: a ring
gear to which a driving force is inputted from a driving source so
as to be rotated; a differential case configured to be rotated
together with the ring gear; a pinion shaft installed on the
differential case, the pinion shaft being configured to be rotated
together with the differential case; pinion gears pivotally mounted
on the pinion shaft; and side gears configured to be meshed with
the pinion gears; wherein: the differential case includes: a first
differential case on a side ranging from a pinion-shaft
installation part on which the pinion shaft is installed, to the
ring gear; and a second differential case on a side opposite to the
side ranging from the pinion-shaft installation part on which the
pinion shaft is installed, to the ring gear; the first differential
case is integrally molded from only a low-carbon steel containing
less than 0.45% of C, by forging or by forging and cutting; the
second differential case is integrally molded from only a
low-carbon steel containing less than 0.45% of C, by forging or by
forging and cutting; and the first differential case and the second
differential case are bonded to each other by welding.
According to the present invention, since no bolt is required to
secure the first differential case and the second differential case
to each other, thicknesses of the first differential case and the
second differential case can be reduced, to thereby reduce a weight
of the differential case. Further, since the number of components
(bolts) can be reduced, a cost reduction can be achieved.
Furthermore, as the first differential case, a part on a side
ranging from the pinion-shaft installation part to the ring gear
(including the ring gear) is made of only a low-carbon steel
containing less than 0.45% of C, and is integrally molded by
forging. In addition, a welding part between the first differential
case and the second differential case is located outside a torque
transmission area (area between the pinion-shaft installation part
and the ring gear). Therefore, the welding part that is poor in
strength can be prevented from being damaged when a torque is
transmitted. Moreover, since the first differential case containing
less than 0.45% of C and the second differential case containing
less than 0.45% of C are welded to each other, there is little
possibility that the welding part hardens to generate a crack.
Therefore, there can be provided the differential gear having a
desired durability.
Preferably, the first differential case and the pinion shaft are
secured to each other by caulking. This securing method is
significantly simple. In particular, since the first differential
case is integrally molded from only the low-carbon steel containing
less than 0.45% of C, by forging or by forging and cutting, the
first differential case has an excellent tenacity. Thus, when the
pinion shaft is secured thereto by caulking, breakdown of the first
differential case can be avoided. In this case, preferably, the
pinion shaft is molded from only a low-carbon steel containing less
than 0.45% of C. In this case, since the pinion shaft also has an
excellent tenacity, when the pinion shaft is secured to the first
differential case by caulking, breakdown of the pinion shaft can be
avoided.
For example, the first differential case is integrally molded from
only a low-carbon steel whose content of C is between 0.10% and
0.40%, by forging or by forging and cutting. Similarly, for
example, the second differential case is integrally molded from
only a low-carbon steel whose content of C is between 0.10% and
0.40%, by forging or by forging and cutting.
The above consideration is mainly given to carbon steels, whose
characteristics are mostly determined by an amount of C (carbon).
The inventors of the present invention further studied other
structured steels, whose characteristics may be affected by an
amount of any other component than C (carbon). As a result, the
inventors have found that Carbon Equivalent, instead of the amount
of C, should be used as the standard for such structured
steels.
The Carbon Equivalent is defined as follows in JIS (Japanese
Industrial Standards). Carbon Equivalent
(Ceq)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 Herein, C is the amount of
carbon (%), Mn is the amount of manganese (%), Si is the amount of
silicon (%), Ni is the amount of nickel (%), Cr is the amount of
chromium (%), Mo is the amount of molybdenum (%), and V is the
amount of vanadium (%).
Then, the threshold "0.45%" of the amount of carbon, which was
firstly found by the inventors, can be converted into "0.60%" of
the Carbon Equivalent in case of the general carbon steels (for
example, S45C), by using the component data thereof (An example of
component data of S45C: C=0.46, Mn=0.72, Si=0.18, Ni=0.04, Cr=0.11,
V=0.00: the amount of C=0.46%, Carbon Equivalent=0.61%). The
inventors have confirmed that such conversion (replacement) is
actually applicable to the present invention for not only the
general carbon steels but also the other structured steels.
That is, the present invention is a differential gear comprising: a
ring gear to which a driving force is inputted from a driving
source so as to be rotated; a differential case configured to be
rotated together with the ring gear; a pinion shaft installed on
the differential case, the pinion shaft being configured to be
rotated together with the differential case; pinion gears pivotally
mounted on the pinion shaft; and side gears configured to be meshed
with the pinion gears; wherein: the differential case includes: a
first differential case on a side ranging from a pinion-shaft
installation part on which the pinion shaft is installed, to the
ring gear; and a second differential case on a side opposite to the
side ranging from the pinion-shaft installation part on which the
pinion shaft is installed, to the ring gear; the first differential
case is integrally molded from only a first structured steel
containing less than 0.60% of Carbon Equivalent, by forging or by
forging and cutting; the second differential case is integrally
molded from only a second structured steel containing less than
0.60% of Carbon Equivalent, by forging or by forging and cutting;
and the first differential case and the second differential case
are bonded to each other by welding.
According to the present invention, since no bolt is required to
secure the first differential case and the second differential case
to each other, thicknesses of the first differential case and the
second differential case can be reduced, to thereby reduce a weight
of the differential case. Further, since the number of components
(bolts) can be reduced, a cost reduction can be achieved.
Furthermore, as the first differential case, a part on a side
ranging from the pinion-shaft installation part to the ring gear
(including the ring gear) is made of only a first structured steel
containing less than 0.60% of Carbon Equivalent, and is integrally
molded by forging, or forging and cutting. In addition, a welding
part between the first differential case and the second
differential case is located outside the torque transmission area
(area between the pinion-shaft installation part and the ring
gear). Therefore, the welding part that is poor in strength can be
prevented from being damaged when a torque is transmitted.
Moreover, since the first differential case made of the first
structured steel containing less than 0.60% of Carbon Equivalent
and the second differential case made of a second structured steel
containing less than 0.60% of Carbon Equivalent are welded to each
other, there is little possibility that the welding part hardens to
generate a crack. Therefore, there can be provided the differential
gear having a desired durability.
In a case where the first structured steel and the second
structured steel are bonded by fusion welding, if the sum of a
value of Hot Crack Sensitivity of the first steel part and a value
of Hot Crack Sensitivity of the second steel part is less than 7.0,
generation of a crack may be prevented more surely. The value of
Hot Crack Sensitivity is calculated in accordance with the
following expression. Hot Crack Sensitivity
(HCS)=1000.times.C(S+P+Si/25+Ni/100)/(3Mn+Cr+Mo+V) Herein, C is the
amount of carbon (%), S is the amount of sulfur (%), P is the
amount of phosphorus (%), Si is the amount of silicon (%), Ni is
the amount of nickel (%), Mn is the amount of manganese (%), Cr is
the amount of chromium (%), Mo is the amount of molybdenum (%), and
V is the amount of vanadium (%).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a differential gear in one embodiment
of the present invention.
FIG. 2A is a sectional view of a first material, for explaining a
molding step of a first differential case of the differential gear
in one embodiment of the present invention.
FIG. 2B is a sectional view of a preparatory first differential
case, for explaining the molding step of the first differential
case of the differential gear in one embodiment of the present
invention.
FIG. 3 is a sectional view of the first differential case of the
differential gear in one embodiment of the present invention.
FIG. 4A is a sectional view of a second material, for explaining a
molding step of a second differential case of the differential gear
in one embodiment of the present invention.
FIG. 4B is a sectional view of a preparatory second differential
case, for explaining the molding step of the second differential
case of the differential gear in one embodiment of the present
invention.
FIG. 4C is a sectional view of the second differential case of the
differential gear in one embodiment of the present invention.
FIG. 5A is a sectional view of a third material, for explaining a
molding step of a pinion shaft of the differential gear in one
embodiment of the present invention.
FIG. 5B is a sectional view of a preparatory pinion shaft, for
explaining the molding step of the pinion shaft of the differential
gear in one embodiment of the present invention.
FIG. 6A is a sectional view of the pinion shaft of the differential
gear in one embodiment of the present invention.
FIG. 6B is a plan view of the pinion shaft of the differential gear
in one embodiment of the present invention.
FIG. 7 is a sectional view of an apparatus for caulking the first
differential case and the pinion shaft.
FIG. 8A is a plan view of a main part of a pinion-shaft
installation part of the first differential case, before the pinion
shaft is caulked and secured thereto.
FIG. 8B is a plan view of the main part of the pinion-shaft
installation part of the first differential case, after the pinion
shaft has been caulked and secured thereto.
FIG. 9 is a sectional view for explaining a method of assembling
the first differential case and the second differential case.
FIG. 10 shows a data table of structured steels including general
carbon steels.
FIG. 11 shows a result of evaluation as structured steels for
fusion welding, regarding the structured steels shown in FIG.
10.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a sectional view of a differential gear in one embodiment
of the present invention.
FIG. 2A is a sectional view of a first material, for explaining a
molding step of a first differential case of the differential gear
in one embodiment of the present invention. FIG. 2B is a sectional
view of a preparatory first differential case, for explaining the
molding step of the first differential case of the differential
gear in one embodiment of the present invention. FIG. 3 is a
sectional view of the first differential case of the differential
gear in one embodiment of the present invention. FIG. 4A is a
sectional view of a second material, for explaining a molding step
of a second differential case of the differential gear in one
embodiment of the present invention.
FIG. 4B is a sectional view of a preparatory second differential
case, for explaining the molding step of the second differential
case of the differential gear in one embodiment of the present
invention. FIG. 4C is a sectional view of the second differential
case of the differential gear in one embodiment of the present
invention. FIG. 5A is a sectional view of a third material, for
explaining a molding step of a pinion shaft of the differential
gear in one embodiment of the present invention. FIG. 5B is a
sectional view of a preparatory pinion shaft, for explaining the
molding step of the pinion shaft of the differential gear in one
embodiment of the present invention. FIG. 6A is a sectional view of
the pinion shaft of the differential gear in one embodiment of the
present invention. FIG. 6B is a plan view of the pinion shaft of
the differential gear in one embodiment of the present
invention.
FIG. 7 is a sectional view of an apparatus for caulking the first
differential case and the pinion shaft. FIG. 8A is a plan view of a
main part of a pinion-shaft installation part of the first
differential case, before the pinion shaft is caulked and secured
thereto. FIG. 8B is a plan view of the main part of the
pinion-shaft installation part of the first differential case,
after the pinion shaft has been caulked and secured thereto. FIG. 9
is a sectional view for explaining a method of assembling the first
differential case and the second differential case.
At first, a differential gear 1 is described with reference to FIG.
1. A differential case 2 of the differential gear 1 is composed of
a first differential case 2a and a second differential case 2b.
The first differential case 2a is made of only a low-carbon steel
(first structured steel) containing less than 0.45% of C, and has
on one axial end thereof a first boss part 13 that is axially
extended. A first through-hole 15 is formed inside the first boss
part 13. Communicated with the first through-hole 15 is a first
gear chamber 17 whose diameter is larger than that of the first
through-hole 15. Communicated with the first gear chamber 17 is a
pinion-shaft installation part 10 on which a pinion shaft 3 is
installed. In addition, there is provided a ring gear 9 which
projects in an outer radial direction about an axis line X. In
addition, in an outer circumference on the other axial end, there
is formed a fitting recess 11 in which the second differential case
2b is fitted.
The second differential case 2b is also made of only a low-carbon
steel (second structured steel) containing less than 0.45% of C,
and has on the other axial end thereof a second boss part 14 that
is axially extended. A second through-hole 18 is formed inside the
second boss part 14. Communicated with the second through-hole 18
is a second gear chamber 20 whose diameter is larger than that of
the second through-hole 18. On an outer circumference on the one
axial end, there is formed a fitting projection 12 to which the
fitting recess 11 of the first differential case 2a is fitted. The
fitting recess 11 and the fitting projection 12 are bonded to each
other by any of electron beam welding, laser welding, and
resistance welding. In this manner, the difference case 2 is
structured.
The content C in the first structured steel is between 0.10% and
0.40%, for example. The content C in the second structured steel is
also between 0.10% and 0.40%, for example.
In the differential case 2, there are disposed: the pinion shaft 3
configured to be rotated together with the differential case 2;
pinion gears 21 and 22 pivotally mounted on the pinion shaft 3;
thrust washers for pinion gear 25 and 26 disposed between the
respective pinion gears 21 and 22 and the differential case 2; side
gears 23 and 24 configured to be meshed with the pinion gears 21
and 22; and thrust washers for side gear 27 and 28 disposed between
the respective side gears 23 and 24 and the differential case 2.
The content C in a structured steel forming the pinion shaft is
also between 0.10% and 0.40%, for example.
Next, a method of forming the first differential case 2a is
described with reference to FIGS. 2A to 3.
At first, from a cylindrical first material 40, shown in FIG. 2A,
which is a low-carbon steel (e.g., SCM420 and S35C) containing less
than 0.45% of C (preferably, between 0.10% and 0.40%), a
preparatory first differential case 42a, shown in FIG. 2B, is
molded by a plastic deformation by hot forging.
As shown in FIG. 2B, the preparatory first differential case 42a
has on one axial end thereof the first boss part 13 that is axially
extended. A preparatory first through-hole 43 is formed inside the
first boss part 13. Communicated with the preparatory first
through-hole 43 is a preparatory first gear chamber 47 whose
diameter is larger than that of the preparatory first through-hole
43. In addition, there is formed a preparatory ring gear 49 which
projects in the outer radial direction about the axis line X. In
addition, in the outer circumference on the other axial end, there
is formed a preparatory fitting recess 41.
As shown in FIG. 3, the preparatory first through-hole 43 of the
preparatory first differential case 42a is bored and finished, and
a first lubrication groove 16 is formed by cutting, so that the
first through-hole 15 is provided. The preparatory first gear
chamber 47 is cut in conformity to shapes of the thrust washers for
pinion gear 25 and 26 and a shape of the thrust washer for side
gear 27, so that the first gear chamber 17 is provided. Further,
the pinion-shaft installation part 10 is formed by cutting between
the outer circumference on the other axial end of the preparatory
first differential case 42a and the first gear chamber 17. A
caulking recess 10a is formed by cutting in an outer diametrical
end of the pinion-shaft installation part 10. The preparatory ring
gear 49 and the preparatory fitting recess 41 are respectively cut
into required shapes, so that the ring gear 9 and the fitting
recess 11 are provided.
The preparatory first differential case 42a, which has been
subjected to the cutting process, is then carburized and quenched,
whereby a hardness thereof is increased.
Due to the above steps, the first differential case 2a having a
desired hardness can be completed as an integral mold piece, as a
whole, which is formed by forging. An area from the ring gear 9 to
the pinion-shaft installation part 10, which is a torque
transmission area, is seamless in terms of material.
Next, a method of forming the second differential case 2b is
described with reference to FIGS. 4A to 4C.
At first, from a cylindrical second material 50, shown in FIG. 4A,
which is a low-carbon steel (e.g., SCM420 and S35C) containing less
than 0.45% of C (preferably, between 0.10% and 0.40%), a
preparatory second differential case 52b, shown in FIG. 4B, is
molded by a plastic deformation by hot forging.
As shown in FIG. 4B, the preparatory second differential case 52b
has on one axial end thereof the second boss part 14 that is
axially extended. A preparatory second through-hole 58 is formed
inside the second boss part 14. Communicated with the preparatory
second through-hole 54 is a preparatory second gear chamber 57
whose diameter is larger than that of the preparatory second
through-hole 53. In the outer circumference on the other axial end,
there is formed a preparatory fitting projection 52.
As shown in FIG. 4C, the preparatory second through-hole 53 of the
preparatory second differential case 52b is bored and finished, and
a second lubrication groove 19 is formed by cutting, so that the
second through-hole 18 is provided. The preparatory second gear
chamber 57 is cut in conformity to shapes of the thrust washers for
pinion gear 25 and 26 and a shape of the thrust washer for side
gear 28, so that the second gear chamber 20 is provided. In
addition, the preparatory fitting projection 52 is cut into a
required shape, so that the fitting projection 12 is provided.
Due to the above steps, the second differential case 2b having a
desired hardness can be completed as an integral mold piece, as a
whole, which is formed by forging. Since a required hardness of the
second differential case 2b is lower than that of the first
differential case 2a, the second differential case 2b is not
subjected to a carburizing and quenching process.
Next, a method of forming the pinion shaft 3 is described with
reference to FIGS. 5A to 6B.
At first, from a cylindrical third material 60, shown in FIG. 5A,
which is a low-carbon steel (e.g., SCM415) containing less than
0.45% of C (preferably, between 0.10% and 0.40%), a preparatory
pinion shaft 63, shown in FIG. 5B, is molded by a plastic
deformation by cold forging.
As shown in FIG. 5B, the preparatory pinion shaft 63 has a diameter
smaller than that of the third material 60, and has an axial length
longer than that of the third material 60. In addition, the
preparatory pinion shaft 63 has a lubrication surface 3c at a
position in opposition to a pinion gear hole 21a of the pinion gear
21, and a lubrication surface 3e at a position in opposition to a
pinion gear hole 22a of the pinion gear 22.
Following thereto, in order to increase a hardness thereof, the
preparatory pinion shaft 63 is carburized and quenched. Thereafter,
as shown in FIGS. 6A and 6B, pinion shaft recesses 3a and 3b are
formed by cutting in opposed axial ends. Further, outer
circumferences of the opposed axial ends are cut (carburized and
quenched portions of the opposed axial ends of the pinion shaft 3
are removed). Due to the above steps, the pinion shaft 3 having a
desired shape and a desired hardness can be formed.
Next, a caulking apparatus 30 for installing the pinion shaft 3 on
the first differential case 2a is described with reference to FIG.
7. The caulking apparatus 30 has on its center part a support table
31 on which the first differential case 2a can be placed. The
support table 31 is supported by columns 32. The caulking apparatus
30 is equipped with an upper caulking punch 33 and a lower caulking
punch 34, for caulking the opposed ends of the pinion shaft 3 to
the pinion shaft installation part 10 of the first differential
case 2a. A spring 35 is disposed around each of the columns 32,
whereby the support table 31 is located at a substantially
intermediate position of the column 32 by an elastic force of the
spring 35.
A method of assembling the differential gear 1 is described with
reference to FIG. 7, FIGS. 8A and 8B, and FIG. 9.
As shown in FIG. 7, the side gear 23 in which the thrust washer for
side gear 27 has been attached on a side away from a tooth profile
thereof is placed on a rotation axis X of the first differential
case 2a such that a tooth-profile side of the side gear 23 faces a
center Y of the differential case.
Then, the pinion gear 21 in which the thrust washer for pinion gear
25 has been attached on a side away from a tooth profile thereof,
and the pinion gear 22 in which the thrust washer for pinion gear
26 has been attached on a side away from a tooth profile thereof,
are opposedly positioned to each other over the rotation axis X of
the differential case, such that the respective pinion gears 21 and
22 are meshed with the side gear 23. At this time, the pinion gear
holes 21a and 22 of the respective pinion gears 21 and 22, the
pinion-shaft installation part 10 of the first differential case
2a, and holes of the respective thrust washers for pinion gear 25
and 26 are aligned to each other.
Subsequently, the pinion shaft 3 is inserted through the
pinion-shaft installation part 10, the hole of the thrust washer
for pinion gear 25, and the pinion gear hole 21a formed in the
pinion gear 21, to reach the rotation axis X of the first
differential case 2a. The pinion shaft 3 is further guided to the
pinion gear hole 22a of the pinion gear 22, the hole of the thrust
washer for pinion gear 26, and the pinion-shaft installation part
10, which are in symmetrical with respect to the rotation axis X.
Then, the pinion shaft 3 is positioned at a predetermined location
of the first differential case 2a. At this time, as shown in FIG.
8A, the opposed ends of the pinion shaft 3 are merely in contact
with the pinion-shaft installation part 10 of the first
differential case 2a, and thus the pinion shaft 3 is movable with
respect to the first differential case 2a.
The first differential case 2a, in which the pinion gears 21 and
22, the thrust washers for pinion gear 25 and 26, the side gear 23,
and the pinion shaft 3 have been arranged in position, is placed on
the support table 31 of the caulking apparatus 30. At this time, in
order that the pinion shaft 3 does not drop out of the first
differential case 2a, the one axial end of the pinion shaft 3 is
supported by the lower caulking punch 34.
After that, the upper caulking punch 33 disposed above the support
table 31 is moved downward. The support table 31 is located at the
substantially intermediate positions of the columns 32 by the
elastic forces of the springs 5 disposed around the columns 32 of
the support table 31. However, when a distal end of the upper
caulking punch 33 presses down the pinion shaft recess 3a of the
pinion shaft 3, the first differential case 2a placed on the
support table 31 and the support table 31 are moved downward, while
the springs 35 are contracted.
Then, the distal end of the upper caulking punch 33 is pressed into
the pinion shaft recess 3a, and a distal end of the lower caulking
punch 34 is pressed into the pinion shaft recess 3b. At this time,
as show in FIG. 8B, by the upper caulking punch 33 and the lower
caulking punch 34, the pinion shaft recesses 3a and 3b are
plastically deformed into installation recesses 10a of the
pinion-shaft installation part 10 of the first differential case
2a. Namely, a caulking process is performed. Herein, since the
carburized and quenched portions of the opposed axial ends of the
pinion shaft 3 have been removed in advance by cutting, there is no
possibility that the pinion shaft recesses 3a and 3b are cracked,
when the pinion shaft recesses 3a and 3b are plastically deformed
into the installation recesses 10a.
After the caulking process has been finished, the upper caulking
punch 33 is moved upward, so that the first differential case 2a
placed on the support table 31 and the support table 31 are
returned to the substantially intermediate position of the columns
32 by restoring forces of the springs 35. In this manner, securing
of the first differential case 2a and the pinion shaft 3 by
caulking is finished.
Thereafter, as shown in FIG. 9, the side gear 24 in which the
thrust washer for side gear 28 has been attached on a side away
from a tooth profile thereof is placed such that the side gear 24
is meshed with the pinion gears 21 and 22.
Then, the fitting recess 11 of the first differential case 2a and
the fitting projection 12 of the second differential case 2b are
fitted to each other. Subsequently, the fitting recess 11 and the
fitting projection 12 are fusion-bonded by electron beam welding,
whereby assemblage of the differential case 2 of the differential
gear 1 is finished.
According to the above differential case 2, since no bolt is
required to secure the first differential case 2a and the second
differential case 2b to each other, thicknesses of the first
differential case 2a and the second differential case 2b can be
reduced, to thereby reduce a weight of the differential case 2.
Further, since the number of components (bolts) can be reduced, a
cost reduction can be achieved. Furthermore, as the first
differential case 2a, a part on a side ranging from the
pinion-shaft installation part 10 to the ring gear 9 (including the
ring gear 9) is made of only the low-carbon steel containing less
than 0.45% of C, and is integrally molded by forging. In addition,
a welding part between the first differential case 2a and the
second differential case 2b is located outside the torque
transmission area (area between the pinion-shaft installation part
10 and the ring gear 9). Therefore, the welding part that is poor
in strength can be prevented from being damaged when a torque is
transmitted. Moreover, since the first differential case 2a
containing less than 0.45% of C and the second differential case 2b
containing less than 0.45% of C are welded to each other, there is
little possibility that the welding part hardens to generate a
crack. Therefore, there can be provided the differential gear 1
having a desired durability.
In addition, since the first differential case 2a and the pinion
shaft 3 are secured to each other by caulking, the first
differential case 2a and the pinion shaft 3 can be secured to each
other in a significantly simple manner. In particular, since the
first differential case 2a is made of only the low-carbon steel
containing less than 0.45% of C and is integrally molded by forging
as a whole, the first differential case 2a has an excellent
tenacity. Therefore, when the pinion shaft 3 is secured thereto by
caulking, breakdown of the first differential case 2a can be
avoided.
In addition, the pinion shaft 3 is also molded from only the
low-carbon steel containing less than 0.45% of C. Thus, since the
pinion shaft 3 also has an excellent tenacity, when the pinion
shaft 3 is secured to the first differential case 2a by caulking,
breakdown of the pinion shaft 3 can be avoided.
In the above embodiment, molding of the first differential case 2a
and the second differential case 2b is performed by hot forging.
However, cold forging and warm forging may be used.
In addition, the bonding operation by electron beam welding is
taken as an example of fusing and bonding the first differential
case 2a and the second differential case 2b. However, another
fusion welding, such as laser welding and resistance welding, may
be used.
In addition, the positioning upon welding is performed by means of
the fitting recess 11 formed in the first differential case 2a and
the fitting projection 12 formed on the second differential case
2b. However, the positioning may be performed by means of a fitting
projection formed on the first differential case 2a and a fitting
recess formed in the first differential case 2b.
Further, in the above embodiment, the pinion shaft 3 and the first
differential case 2a are secured to each other by caulking by
plastically deforming the opposed ends of the pinion shaft 3 into
the first differential case 2a. However, the pinion shaft 3 and the
first differential case 2a may be secured to each other by caulking
by plastically deforming the first differential case 2a into the
opposed ends of the pinion shaft 3.
In addition, in order to prevent the welding part from hardening to
generate a crack, the first differential case 2a and the second
differential case 2b are made of the low-carbon steel containing
less than 0.45% of C, which is an important feature of the present
invention. Preferably, the content of C is between 0.10% and 0.40%
in the light of preventing hardening of the welding part. More
preferably, the content of C is between 0.10% and 0.35% in the
light of preventing hardening of the welding part.
The above explanation is mainly given to carbon steels, whose
characteristics are mostly determined by an amount of C (carbon).
The inventors of the present invention further studied other
structured steels, whose characteristics may be affected by an
amount of any other component than C (carbon). As a result, the
inventors have found that Carbon Equivalent, instead of the amount
of C, should be used as the standard for such structured
steels.
The Carbon Equivalent is defined as follows in JIS (Japanese
Industrial Standards). Carbon Equivalent
(Ceq)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 Herein, C is the amount of
carbon (%), Mn is the amount of manganese (%), Si is the amount of
silicon (%), Ni is the amount of nickel (%), Cr is the amount of
chromium (%), Mo is the amount of molybdenum (%), and V is the
amount of vanadium (%).
Then, the threshold "0.45%" of the amount of carbon, which was
firstly found by the inventors, can be converted into "0.60%" of
the Carbon Equivalent in case of the general carbon steels (for
example, S45C), by using the component data thereof (An example of
component data of S45C: C=0.46, Mn=0.72, Si=0.18, Ni=0.04, Cr=0.11,
V=0.00: the amount of C=0.46%, Carbon Equivalent=0.61%). The
inventors have confirmed that such conversion (replacement) is
actually applicable to the present invention for not only the
general carbon steels but also the other structured steels.
Namely, in comparison with the above embodiment, in place of the
low-carbon steel containing less than 0.45% of C, there may be used
a structured steel containing less than 0.60% of carbon
equivalent.
Also in this case, since no bolt is required to secure the first
differential case 2a and the second differential case 2b to each
other, thicknesses of the first differential case 2a and the second
differential case 2b can be reduced, to thereby reduce a weight of
the differential case 2. Further, since the number of components
(bolts) can be reduced, a cost reduction can be achieved.
Furthermore, as the first differential case 2a, a part on a side
ranging from the pinion-shaft installation part 10 to the ring gear
9 (including the ring gear 9) is made of only a first structured
steel containing less than 0.60% of Carbon Equivalent, and is
integrally molded by forging, or forging and cutting. In addition,
a welding part between the first differential case 2a and the
second differential case 2b is located outside the torque
transmission area (area between the pinion-shaft installation part
10 and the ring gear 9). Therefore, the welding part that is poor
in strength can be prevented from being damaged when a torque is
transmitted. Moreover, since the first differential case 2a made of
the first structured steel containing less than 0.60% of Carbon
Equivalent and the second differential case 2b made of a second
structured steel containing less than 0.60% of Carbon Equivalent
are welded to each other, there is little possibility that the
welding part hardens to generate a crack. Therefore, there can be
provided the differential gear 1 having a desired durability.
Furthermore, when the first structured steel and the second
structured steel are bonded by fusion welding, if the sum of a
value of Hot Crack Sensitivity of the first structured steel and a
value of Hot Crack Sensitivity of the second structured steel is
less than 7.0, generation of a crack may be prevented more surely.
The value of Hot Crack Sensitivity is calculated in accordance with
the following expression. Hot Crack Sensitivity (HCS)=1000.times.C
(S+P+Si/25+Ni/100)/(3Mn+Cr+Mo+V) Herein, C is the amount of carbon
(%), S is the amount of sulfur (%), P is the amount of phosphorus
(%), Si is the amount of silicon (%), Ni is the amount of nickel
(%), Mn is the amount of manganese (%), Cr is the amount of
chromium (%), Mo is the amount of molybdenum (%), and V is the
amount of vanadium (%).
FIG. 10 shows a data table of structured steels including general
carbon steels. In the table, Ceq means the Carbon Equivalent, and
HCS means the Hot Crack Sensitivity. FIG. 11 shows a result of
evaluation as structured steels for fusion welding (first
differential case 2a and second differential case 2b), regarding
the structured steels shown in FIG. 10.
As shown in FIG. 9, in the evaluation as structured steels for
fusion welding, "0.45%" of the amount of carbon and "0.60%" of the
Carbon Equivalent were threshold of the eligibility. In addition,
when the sum of values of Hot Crack Sensitivity of two structured
steels to be fusion welded is less than 7.0, it was actually
confirmed that generation of a crack may be prevented more
surely.
It is needless to say that workability is also taken into
consideration in selecting structured steel(s). Some structured
steels may not be used if they have poor workability.
* * * * *